Small signal analysis of ultra-wide bandgap Al0.7Ga0.3N channel MESFETs

We report RF small signal analysis of ultra-wide bandgap AlGaN channel MESFETs with Al composition of 70%. Ohmic contacts are achieved using a linearly-graded and heavily-doped AlGaN layer grown by metal organic chemical vapor deposition. The device with a gate length of 250 nm shows a drain current density (I-DSS) of 370 mA/mm when the gate is shorted to source, unity current gain cutoff frequency (f(T)) of 8.8 GHz, and maximum oscillating frequency (f(MAX)) of 15 GHz. Bias-dependent device parameters are extracted from measured S-parameters based on a small-signal equivalent circuit model. The peak effective electron velocity of 2.9 x 10(6) cm/s is obtained at V-GS = - 7 V. We show that the average electron velocity (v(e)) under the gate limits the device intrinsic RF performance while the high source access resistance (R-s) and drain access resistance (R-d) limits the device extrinsic RF performance.